The present invention relates to a method for testing a conductor element, applicable to locating a continuity defect of the conductor element. The present invention relates, in particular, to testing the electrical continuity of the metal screen of a telecommunication cable.
Current telecommunication networks, that have a tree structure, are produced by means of cables 1 with a large section of the type represented in FIG. 1A, comprising several hundred or thousand electric wires 2 insulated from each other by a suitable wrapping and arranged two by two to form telephone pairs. The assembly is protected from electric disturbance by a metal sheath, or screen 3, that is itself covered by a protective sheath 4 made of an electrically insulating material such as polyethylene, PVC, . . .
Such telecommunication cables, arranged in the ground or in the air, are subjected to various attacks the most frequent of which are caused by lightning, rodents, road works, the rubbing of tree branches . . . These various attacks can lead to a tear 5 of the protective sheath and to the penetration of water into the cable. From an electrical point of view, such deterioration results in an insulation defect of the screen 3 relative to the ground, represented in a diagram in FIG. 1B by a resistance Rd, and leads to the occurrence of a voltage Vc, or xe2x80x9cscreen potentialxe2x80x9d, generated, in particular, by the combination of the metal of the screen 3 with water and various metal oxides. When a tightness defect is not repaired in time, the deterioration of the cable extends to the wrapping of the electric wires and develops over a substantial cable length due to the spread of water.
Therefore, testing the proper tightness of cables is a major concern for telecommunication operators and more and more needs to be automated with a view to reducing maintenance costs and working time on site.
Another test that must be performed on networks of telecommunication cables is that of the electrical continuity of screens, that are generally connected to the ground at their ends to allow the flow of electric charges brought about by electromagnetic disturbance or rises in the electric potential in the ground. The testing of the electrical continuity of screens between two connection points to the ground is also currently a major concern for telecommunication operators, as the latter must guarantee optimal performances of their networks due to the increase in the rates of digital data transfer imposed by the development of the Internet network and other computer applications.
In recent years, the applicant has designed, developed and perfected an automatic surveillance system for telecommunication networks constituted by a set of measuring devices marketed under the reference xe2x80x9cIMDxe2x80x9d (Remote Measuring Interface). Such devices, described in the patent EP 408 480 and in the international application PCT/FR99/02288, are arranged at the connection points to the ground of the metal screens and linked by telephone pairs to a piece of local maintenance equipment, that is itself linked to a regional maintenance centre. They allow various tests and measuring operations to be conducted daily, in particular:
the disconnection of the screens relative to the ground and the measuring of the insulation resistance of the screens and of the screen potential,
the detection and location of an insulation defect of a screen by injecting a low frequency voltage, in compliance with a method described in the above-mentioned international application, and
the detection of a continuity defect of a screen, by a method called xe2x80x9cground loopxe2x80x9d.
However, as telecommunication cables are of considerable lengths ranging from approximately one hundred meters to a few kilometres, locating an insulation defect in a segment of cable by means of IMD devices must be completed by a step of precisely locating the defect in the field in order to repair it.
In the above-mentioned international application, the applicant proposed an additional locating method allowing the anomaly affecting a suspect segment of cable to be located on the repair site. As a reminder, this additional locating method consists in injecting into the screen of the suspect portion of cable two currents of different frequencies, measuring the currents at various points of the screen, then calculating the real part of at least one of the two currents to be free from the influence of the leak capacitance of the screen. A sharp drop in the real part of the current at a determined point of the cable allows the insulation defect to be located.
A similar problem arises as far as electrical continuity defects are concerned, as the ground loop method allows a continuity defect to be detected in a segment of network without precisely locating the point of discontinuity. For a better understanding, FIG. 2 schematically represents a network of telecommunication cables comprising a cable xe2x80x9cAxe2x80x9d comprising N wires the end of which is connected by means of a splice box 10 to two cables xe2x80x9cBxe2x80x9d and xe2x80x9cCxe2x80x9d comprising N/2 wires each. The cable B is itself connected by means of a splice box 11 to two cables xe2x80x9cDxe2x80x9d and xe2x80x9cExe2x80x9d comprising N/4 wires each. The screen of cable A is connected to the screens of cables B and C and the screen of cable B is connected to the screens of cables D and E. The screen of cable A is connected to the ground at its point of origin through a device IMD1 and the screens of cables C, D, E are connected to the ground at their end points through devices IMD2, IMD3, IMD4.
When measuring a ground loop, for example in the segment ABE, the device IMD1 maintains the point of origin of the screen A connected to the ground while the devices IMD2, IMD3, IMD4 disconnect the end points of the screens C, D, E. The device IMD4 measures the resistance of the loop constituted by the resistance of the screens ABE, the ground resistance of the device IMD1 and its own ground resistance. If the loop resistance is very high, that may mean that the segment ABE has a continuity defect. However, the location of the continuity defect is unknown. The defect may for example be situated in one of the connection boxes 10, 11, and must be located to carry out the repair.
Now, to the applicant""s knowledge, no really satisfactory method has yet been proposed to locate the continuity defect of a metal screen in the field, current methods being essentially based on a visual inspection.
The present invention aims to overcome this inconvenience.
More particularly, one general object of the present invention is to provide a method for testing a conductor element applicable to locating an electrical continuity defect, that is precise, reliable and simple to implement.
Another object of the present invention is to provide a device for testing a conductor element applicable to locating an electrical continuity defect, that is small in size and easy to use.
To achieve this object, the present invention is based on the observation that the additional locating method described in the above-mentioned international application is also applicable to locating continuity defects, by calculating the imaginary part of the bifrequency current instead of calculating the real part. A purely capacitive current value is obtained which allows a continuity defect to be located with great precision.
More particularly, the present invention provides a method for testing a conductor element applicable to locating a continuity defect of the conductor element, the conductor element having, relative to a reference conductor, an insulation resistance and a leak capacitance spread along the conductor element, the method comprising a step of injecting into the conductor element at least two currents of different frequencies by means of a current or voltage generator, one terminal of which is connected to the reference conductor, at least one step of measuring the amplitudes of currents at one measuring point chosen along the conductor element, and a step of calculating the imaginary part of at least one of the currents and/or of calculating the leak capacitance of the conductor element downstream from the measuring point, using the measured amplitudes of currents.
According to one embodiment, the method comprises a plurality of steps of measuring the amplitudes of currents at various measuring points, and a plurality of steps of calculating, after each measurement, the imaginary part of at least one of the currents and/or the downstream leak capacitance, an electrical continuity defect of the conductor element being located when the imaginary part calculated and/or the downstream leak capacitance proves to be zero before the end of the conductor element is reached.
The present invention also provides an application of the method above to locating an electrical insulation defect of the conductor element relative to the reference conductor, the method comprising a step of calculating the real part of at least one of the currents and/or the insulation resistance downstream from a measuring point.
According to one embodiment, the method comprises a plurality of steps of measuring the amplitudes of currents at various measuring points, and a plurality of steps of calculating, after each measurement, the real part of at least one of the currents and/or the downstream insulation resistance, an electrical insulation defect being located when the real part calculated shows a clear attenuation and/or when the downstream insulation resistance rapidly rises without this being justified by the topography of the conductor element.
According to one embodiment, the method comprises a step of calculating a formula of type: Kx√[Ky|i1|2xe2x88x92Kz|i2|2], in which |i1| and |i2| are the amplitudes of currents and Kx, Ky, Kz are constants.
According to one embodiment, the conductor element comprises several telecommunication cable screens (A-E) connected to each other, and the reference conductor is the ground.
The present invention also relates to a device for testing a conductor element, comprising a connection terminal to a contactless current sensor, an analog/digital conversion circuit of the signal delivered by the current sensor, and a calculation means, in which the calculation means is arranged to analyse the signal delivered by the current sensor and to extract the amplitudes of at least two currents of different frequencies present in the signal delivered, and to calculate the imaginary part of at least one of the currents and/or an electric capacitance by means of a formula of type: K1√[K2|i1|2xe2x88x92K3|i2|2], in which |i1| and |i2| are the amplitudes of currents and K1, K2, K3 are constants.
According to one embodiment, the calculation means is also arranged to calculate the real part of at least one of the currents and/or an electric resistance, by means of a formula of type: K4√[K5|i2|2xe2x88x92K6|i1|2], in which K4, K5 and K6 are constants.
According to one embodiment, the device comprises selection means to choose between the calculation of the imaginary part of at least one of the currents and/or the calculation of an electric capacitance, on the one hand, and the calculation of the real part of at least one of the currents and/or an electric resistance, on the other hand.
According to one embodiment, the device is intended to be connected to a hook-on ammeter, and comprises calibration terminals intended to be interconnected during a calibration phase, and means for injecting onto one of the calibration terminals at least two calibration currents of different frequencies through a standard resistance.